To close bags and the like supplied to a closure machine in the form of an open tube, it has previously been proposed to apply two jaws against the walls of the tube, and move the jaws towards each other. The jaws, in form of a pair, seal the bag walls. For an optimum seal, three parameters are important: time; temperature; application pressure, that is, pressure between the jaws and with which the walls are pressed together.
The first two parameters, time and temperature, can be easily defined. The timing can be controlled by mechanical or electrical timing apparatus, and can be determined by machine construction, or by the use of electronic control apparatus, preferably, with indicators showing the appropriate timing. Likewise, temperature can be readily controlled, for example with electric or steam heaters, and the temperature at the jaws indicated by suitable thermocouples coupled to temperature indicators or other types of thermometers. The third parameter, pressure, is difficult to control and is usually set empirically. The pressure is that which is applied to the foil walls upon sealing, or welding of the walls of the foil or film. Usually, the pressure is set empirically, dependent on the experience of the operator of the machine. The wall thickness of the foil varies; most commercial packaging foil is supplied with fairly substantial tolerance ranges. The application pressure of the jaws towards each other, between which the foils are pinched, should vary in accordance with the thickness of the foil, the type of foil, or the material thereof, and foil composition. While data regarding optimum pressures can be derived, it was heretofore usually customary to adjust the operating pressure in accordance with experience and knowledge of the operator. Optimum engagement pressure, thus, cannot be obtained at all times.